Process for making friction products

ABSTRACT

An improved process for molding fiber-reinforced articles and in particular, those articles which are formed from monofilaments such as metal fibers and processed mineral fibers of the slag type. The fiber-containing molding composition is enclosed in a restraining means, such as a bag, and placed in a mold and formed during which time the bag advantageously deteriorates.

This is a continuation of application Ser. No. 84,354, filed 10/12/79,now U.S. Pat. No. 4,268,467, issued May 19, 1981.

BACKGROUND OF THE INVENTION

This invention relates to an improved process of making heat-resistant,friction products such as brake shoe linings and the like.

For a long time, asbestos fibers have been the dominant fiber used inmaking brake shoe linings and like products. The success of asbestos waslargely due to its excellent heat resistance and other physicalproperties which it imparted to the finished products. The material hadsubstantial additional processing advantages as became readily apparentto those investigators who attempted to find suitable substitutes forasbestos. For example, asbestos fibers can be viewed as bundles whichtend to open up to form additional fibrous surface and help bind anostensibly dry molding composition together. One important way in whichthis characteristic was utilized included a preforming step whereinasbestos-reinforced compositions were compressed to a shaped articlehaving sufficient density and green strength to facilitate itssubsequent handling and processing into a finished product. A number offiber substitutes have been suggested for asbestos. These include suchmaterials such as the slag material disclosed in U.S. Pat. No. 4,150,011to Searfoss and Jones, the basalt rock product disclosed in U.S. Pat.No. 3,896,075; metal fibers; and the like. In general, these fibroussubstituents resisted the traditional preforming techniques, or yieldedpre-molds which were of an inconveniently low green strength, orrequired undesirable modification of the subsequent molding step.

A number of techniques were tried in an attempt to provide a suitableprocess for utilizing the slag fibers. These involved the necessity ofusing relatively slow processing procedures and/or the use of chemicaladditives. In either case, the resulting process was unwieldy andexpensive.

SUMMARY OF THE INVENTION

It is a principal object of the invention to provide an improved processfor molding a heat-resistant, fiber-reinforced, friction product.

It is a particular object of the invention to provide an improvedprocess as described above which is clean, and economical in energyrequirements.

Another object of the invention is to provide a process for molding suchfriction products which eliminates the need for agreen-strength-imparting preforming step before a final molding step.

A further object of the invention is to provide a process comprising anencapsulation step and the automatic thermal disposal of theencapsulating means after its principal function has been performed.

Still another object of the invention is to provide a novel moldedproduct.

Other objects of the invention will be obvious to those skilled in theart on their reading of this disclosure.

The above objects have been achieved by a process whereby a moldingcomposition is placed within a bag, preferably a thermally decomposablebag, and placed, advantageously without a premolding step, into a moldwherein the composition is formed into an article of manufacture. Theprocess has been found to be particularly suitable for just that classof particles, e.g. monofilament fibers, which fail to provide thedesirable particle interaction previously achieved with asbestos. Onereason for this suitability is that the decreased bulk characteristicsof monofilament fibers enable compositions, usually characterized by abulk density of 30 lbs per cubic foot or more, to be fed to the finalmold in a more compact package than would have been possible withasbestos.

Molding pressures and temperatures are selected within the ranges knownto those skilled, to provide the desired physical properties in thefinished articles. However, if the encapsulating material is to bedestroyed in the mold, temperatures should be selected to assure suchdestruction. Temperatures in excess of about 275° F. and above aresuitable for the destruction of thin polyethylene encapsulating bags.Moreover, if it is decided to recycle a bag, a more thermally resistantand mechanically stronger bag material may be selected in order tocontain the composition. Also the mold temperature may be kept below thetemperature at which the bag will thermally destruct. In such a case,the bag should be constructed from (or coated with an effective quantityof) a material that resists sticking, e.g. a polyfluorocarbon such aspolytetrafluoroethylene or one of the polyfluorocarbon-bearingcopolymers known to the art. Such a bag will normally require either anadhesive seal or a heat sealable coating on the closure portion thereofto provide a suitable sealable bag.

The encapsulating polymer is advantageously very thin, e.g. from about0.0005 to 0.003 inch in thickness. It is desirable that the amount ofcomposition encapsulated be weighed with good accuracy, sayplus-or-minus 2% but, preferably, plus-or-minus 0.5%.

The compositions useful in the process of the invention will normallyuse a curable thermosetting resin binder system such as the phenolicsystems, e.g. phenolaldehyde systems well known to the art.

Among the fibers which are readily used are the elongate, spun-blastfurnace slag fibers of the type sold by Jim Walter Corporation, basaltfibers, and metallic fibers. Carbon fibers may also be used althoughthey have a relatively low bulk density. (Carbon fibers are consideredinorganic fibers for the purpose of this description.) Generally,cylindrical fibers which are free of nodular-shaped ends are preferable.The fibers are mixed with the other ingredients, the resulting mix isenclosed (desirably sealed) in the encapsulating web. The encapsulatedcomposition is then transferred to apparatus known in the art as acuring mold. The encapsulating web may be provided in bag form, and thebag will be selected according to the quantity of composition to beincluded and according to the dimensional requirements of the particularmold to be utilized. For example, polyethylene bags of two mils inthickness and about two feet high and eight inches wide are suitable forhandling about 10 pounds of molding composition in the applicationspecifically described in Example 1.

However, such materials as paper may be used when for some reason, theymeet the special requirements of some manufacturer.

SPECIFIC EMBODIMENTS OF THE INVENTION

In this application there is described a preferred embodiment of theinvention and suggested various alternatives and modifications thereof,but it is to be understood that these are not intended to be exhaustiveand that other changes and modifications can be made within the scope ofthe invention. These suggestions herein are selected and included forpurposes of illustration in order that others skilled in the art willmore fully understand the invention and the principles thereof and willbe able to modify it and embody it in a variety of forms, each as may bebest suited in the condition of a particular case.

EXAMPLE 1

A composition is formed of 27% by weight of spun-blast furnace slagfibers of the type available from Jim Walter Corporation; 16% by weightof a thermosetting phenolaldehyde resin system of the type known in thebrake lining art; 13% by weight of small, synthetic rubber particles ofabout 20 mesh and derived from tire peelings; 1% by weight of corundum,and 28% by weight of barytes filler, 5% by weight of flake graphite as aparticulate lubricant, 7% by weight of a friction particle product, i.e.cashew oil/aldehyde reaction product, as known in the brake-liningmanufacture and available from several suppliers and 3% of carbon blackpigment.

The composition is thoroughly mixed as known in the art and 10 pounds ofthe resulting composition is metered into a bag (nominally about 8inches wide and 23 inches high) formed of 0.002-inch thick polyethylenefilm. After the filling operation, the bag is sealed in such a positionthat it holds the composition loosely. A sewing procedure is entirelysuitable as a sealing step, because it is the containment of thecomposition that one is achieving, not a hermetic sealing of the bag.

The loaded and sealed bag is placed in a mold designed to hold a slabfor conversion by procedure known to the art into standard,industry-recognized Part Number FMS 4515 brake shoe lining, all byprocedures as known to the art. The pressure is raised slowly to about700 psi; then a degassing (or bumping) of the mold contents is achievedby relieving the pressure. Thereupon the molding cycle is carried out at330° F. under a pressure of 2000 psi for about 20 minutes. At the end ofthis time, the product is removed from the mold.

The molded slab is then processed through finishing steps, i.e.additional curing and machining steps known in the art of making brakelinings.

The resulting molded product is highly suitable for post-curing andfinishing operations and yields an excellent brake lining characterizedby excellent wear, and a surprisingly beneficial effect on the wear andsurface finish characteristics of opposing metallic brake members.

EXAMPLES 2-3

The process of Example 1 is repeated using bags formed of anotherthermoplastic polymer, i.e. poly(vinylchloride) and, also, of theheat-shrinkable film formed of the copolymer of vinyl chloride andvinylidene chloride and sold under the trademark Saran by the DowChemical Company. Each bag proves to be a highly suitable encapsulatingvehicle and is substantially decomposed during the molding cycle.

The copolymer bag, when subjected to heat and, before being placed inthe mold, shrinks to form a particularly snug preformed package; but anyadvantages in handling of this package in any given mold have to bebalanced against somewhat superior mold-filling characteristics of thosemore loosely encapsulated packages which are not subjected to a heatshrinking step.

EXAMPLE 4

The process of Example 1 is repeated using a Kraft paper bag. Theresultant molded material is acceptable. However, the paper residuepresented some handling and maintenance problems. Moreover, use of asealing step requires either a strip of adhesive activated before theplacing of the bag in the mold or the use of a bag-closing tape.

EXAMPLE 5

The process of Example 1 is repeated using an equivalent volume of thebasalt fiber composition and other ingredients well known in the art.

    ______________________________________                                                              Parts by Weight                                         ______________________________________                                        Basalt fiber            5 parts                                               Phenol/aldehyde Resin Molding Powder                                                                  1 part                                                Chromite Dust           1 part                                                Barytes                 2 parts                                               Brass Dust              1 part                                                ______________________________________                                    

After the composition is molded and the part is treated according toExample 1, a suitable brake shoe results.

EXAMPLES 5-6

The process of Example 1 is repeated substituting volumetricallyequivalent quantities of steel wool fibers and, in a separateexperiment, a volumetric equivalent of monofilament carbon fibers forthe slag fibers. The process is operable for each type of fiber.

It is to be understood that the following claims are intended to coverall of the generic and specific features of the invention hereindescribed and all statements of the scope of the invention which mightbe said to fall therebetween.

What is claimed is:
 1. A shaped article of thermally-resistant,fiber-reinforced, friction material made from a composition comprisingan effective quantity of inorganic heat-resistant fiber reinforcementadmixed with an effective quantity of a thermosetting resin bindersystem and formed by a process comprising the steps of:(a) enclosing aquantity of said composition to be molded to said desired shape within athermally decomposable container of flexible encapsulating material, (b)sealing said container, (c) placing said sealed container and contentstherein within a curing mold having a shape for said shaped article, (d)heating and pressing said composition within said mold to cure saidresin binder to bond the composition into said shaped article whereinsaid heating within said mold thermally decomposes said container; and(e) opening said mold and removing said molded article.
 2. The articleof claim 1 wherein said flexible encapsulating container is athermoplastic bag.
 3. The article of claim 1 wherein said bulk densityof said composition, prior to step (a), is at least 30 lbs. per cubicfoot.
 4. The article of claims 1, 2, or 3, wherein said fiber therein isa spun-slag mineral fiber.
 5. The article of claim 1 or 2 wherein saidfiber is selected from the group consisting of basalt fiber, carbonfiber and steel fiber.
 6. The article of claim 1 wherein said containeris a polymeric film having a thickness of about 0.0005 to 0.0003 inch.